This invention relates to a device for capturing rainfall in both a standard rain gauge and a collector tube incorporating an outflow circuit for simulating runoff. The invention provides an estimate of the rainfall split between soil infiltration and runoff. In addition to measuring the total rainfall, the device estimates the amount of runoff, thereby providing an estimate of the soil infiltration by difference.
Prior art rain gauges measure the total amount of precipitation. This can be helpful to serve as a general indicator of how much total rain has fallen, as well as how often and how much irrigation must be provided for crops, for example. However, the prior art rain gauges fail to take into account how much water has run off from the soil, and how much has soaked in. Of course, the rainfall that runs off serves no benefit in irrigation for crops or other plant growth. There is a need in the art for measuring the amount of rainfall that has run off from the soil and/or the amount that has soaked in.
There are a variety of conventional rain gauges available on the market, including such designs as simple clear collecting tubes with ruler markings, ornate collecting tubes for decorating gardens, non-linear tubes with expanded low-end scales to improve readability and electronic gauges with accuracies to a hundredth of an inch. These devices are all designed to make the same measurement, specifically the amount of rain which falls in the vicinity of the rain gauge.
Generally, when rainfall reaches the ground, it splits into different flow paths. A small amount may be retained on plant surfaces or pond in small depressions along the ground surface. However, the bulk of the rainfall either (1) infiltrates the soil surface or (2) flows overland as runoff. Soil infiltration is of primary interest for agriculture and homeowners with lawns and gardens (who provide supplemental irrigation for plants), while the run off measurements are of interest to hydrologists monitoring lake levels, storm drainage and flood plains, for example. Both groups typically estimate the quantity of interest from the total rainfall using historically developed correlations or simple experience.
Infiltration rates are a function of several variables including soil type, compaction, water content, land slope and plant density, as well as the water depth above the soil. Infiltration rates are highest when the soil is very dry, but as precipitation continues and the soil moisture content increases, the infiltration rate declines and approaches a constant rate termed the soil percolation rate. Whenever the precipitation rate exceeds the infiltration rate, excess water accumulates on the surface and a portion of the excess is lost as runoff. The split to runoff therefore increases with both rainfall quantity and intensity. It is also apparent that the relative fractions of infiltration and runoff experienced with a rain event are not only expected to be site specific, but will also vary for identical rain events (quantity and intensity) based on the effects of recent weather on soil moisture content.
A desirable feature of a device measuring runoff would be design flexibility to fit various formats, including a remotely monitored, self-draining electronic design. The most popular design for electronic rain gauges utilizes a tipping bucket sensor. The sensor is similar to a seesaw with buckets on each end for collecting water. The sensor collects water in the elevated bucket, which becomes heavy upon filling and falls, triggering a signal and dumping the contents. The other side then becomes elevated and repeats the cycle. The tipping bucket sensor is simple, robust and accurate, and is therefore attractive for use in the electronic design for a runoff rain gauge. The device would also desirably include good access for reading the gauge and other monitoring and maintenance for all of the various formats.
When a significant percentage of soil infiltration comes from irrigation, a device measuring runoff would preferably be located inside the irrigation field to capture the effects of irrigation on soil moisture. This will usually require mounting the unit close to grade. A runoff rain gauge that provides a grade level mounting would also enhance accuracy by providing an extended core sample of maximum depth. Although an in situ mounting provides these features, lack of elevation for the core sample could make use of an in situ mounting problematic for a self-draining unit, since a sump pump would be required to return the runoff to grade. An alternate low-level mounting system providing both elevation for runoff drainage to grade and the benefits of an extended core sample is therefore preferred. Ultimately, further design simplification is also desirable.